Tires comprising crown reinforcements have been found to be particularly durable when subjected to high revolution speeds. The overlay ply can, for example, be interposed between the radially outermost belt ply and the tread and consist of a single ply having a width which is about equal to that of the widest of the belt plies. Alternatively, the overlay ply can consist of two separate axially spaced apart ply portions either disposed radially outwardly of the belt such as to cover the edges of the radially outermost belt ply or interposed between the belt plies such as to extend between the edges thereof.
In the following description the overlay ply will be described in terms of a single ply disposed between the belt and the tread, but it will become apparent to a person skilled in the art that problems encountered in the prior art with single overlay plies and the solutions that the present invention presents thereto equally apply to overlay plies consisting of two or more portions as described above.
The overlay ply is usually applied onto the green tire as a single annular layer around the belt with the two end portions of the ply slightly overlapping to form a splice. However, as a result of the expansion of the tire in the curing mold and as a result of the thermal contraction of the textile filaments, the stresses in the cords of the ply are non-uniformly distributed around the circumference of the tire and slippage between the overlay ply end portions tends to occur at their overlap region. This slippage at the overlap region of the overlay ply generally produces an undesired distortion on the underlying belts. This in turn can result in an unbalanced portion in the tire and a reduction in tire uniformity.
It has been proposed in the prior art, to distribute the nonuniformity introduced by the overlay splice over a wider range of the tire circumference and the overlapping area of the overlay ply has been arranged e.g. at an angle of between 30° and 45° to the circumferential direction of the tire, or even at a higher angle (see e.g. LU-A-85964). While this configuration has resulted in a substantial reduction of the distortion of the belt, it has not resulted in a complete elimination thereof.
U.S. Pat. No. 4,325,423 has further proposed to reduce the negative effect on the belts caused by the slippage at the overlap by using an overlap area of a generally zigzag or wavy form extending across the width of the overlay ply. However, the nonuniformity in the stress distribution of the overlay cords still remains localized over a relatively small portion of the circumference of the tire and belt distortion cannot be completely avoided in that portion.
It has also been proposed in the prior art to use overlay plies comprising several circumferentially spaced overlapping sections, thereby allowing the stresses in the cords to be taken up by the overlap portions which are spaced around the circumference of the tire (see e.g. DE-A-2 824 357 and DE-A-2 821 093). However, this solution is costly from a manufacturing point of view as it requires the single turn overlay to be preassembled from several smaller sections.
It has further been proposed in the prior art, e.g. U.S. Pat. No. 4,284,117, to use single yam reinforcing cords for the overlay ply and to wrap several turns of this layer around the circumference of the tire thereby virtually eliminating the possibility of any slippage of the overlay ply in the final tire. Since the ply itself cannot accommodate the expansion of the belt through slippage, the expansion is entirely taken up by the reinforcing yams which to this effect have a relatively low modulus of elongation as compared to the cords used in single layer overlay plies.
In U.S. Pat. No. 4,791,973 a prior art tire is characterized in that the textile reinforcing elements of the overlay ply consist of successively aligned cord sections arranged in parallel rows each having a length which is comprised between ¼ and ⅛ of the corresponding circumferential development of the tire, the cord sections in each row having substantially equal length, and being separated by interruptions of a width such that the total width of the interruptions per circumferentially extending row of cords is less than 4% of the corresponding circumferential development of the tire.
By corresponding circumferential development of the tire is meant throughout this specification the circumference of the overlay ply at the mid-circumferential plane when the tire is mounted on its corresponding rim and inflated to its design pressure, but not subjected to a load.
It is to be noted that U.S. Pat. No. 3,990,493 describes an overlay ply in a radial tire which comprises interruptions or cuts which are regularly spaced around the circumference. However, the discontinuous cord portions have a length which is not greater than 1/10 of the corresponding circumferential development of the tire and is preferably comprised between 1/100 and 4/100 of the corresponding circumferential development of the tire.
It is further to be noted that U.S. Pat. No. 2,945,525 describes an annular layer comprising ring shaped reinforcing elements which have at least one interruption for each complete circumference of the tire. The layer described in this patent is a belt layer which is the sole reinforcement of the crown area of the tire and fulfills the function of the belt which is completely different from the function fulfilled by the overlay ply of the present invention.
It is an object of the present invention to provide a pneumatic tire which is durable at high speeds and which at the same time has a high degree of uniformity and exceptional ride comfort, particularly in the area of low noise generation.
It is a further object of the invention to provide a tire which can be manufactured in a cost-effective manner and which employs overlay plies comprising reinforcement cords with a relatively high modulus of elongation.